ABSTRACT The goal of the NANO (NICU Antibiotics and Outcomes) Trial is to study the longstanding clinical practice of empirically administering intravenous antibiotics to newborn extremely low birthweight (ELBW) infants. ELBW infants commonly receive antibiotics immediately after birth and then for 2-3 subsequent days while clinicians await microbiology culture results. Typically, these culture results are used to guide a decision to continue or discontinue antibiotics based on the presence or absence of a bloodstream infection. The assumption underlying this practice is that early antibiotic administration will improve outcomes in infants undergoing evaluation for early onset sepsis (EOS). However, there are no data to support or refute this assumption, and numerous studies have shown that >98% of ELBW infants receiving empiric antibiotics (EA) do not have EOS. Recent microbiome research has demonstrated that broad-spectrum antibiotic exposure is associated with adverse childhood outcomes, and this may be particularly relevant during a critical window of early bacterial colonization in newborns. We simply do not know whether early EA for ELBW infants improves health outcomes, worsens outcomes, or does not affect outcomes. Data from our group and others document the impact of antibiotics on microbiome development and demonstrate a dose-dependent association between antibiotic days and a composite outcome of late onset sepsis (LOS), necrotizing enterocolitis (NEC), or death in ELBW infants. To formally study whether the incidence of adverse outcomes is higher in infants receiving EA in the first week of life compared to babies receiving placebo, we propose the NANO Trial, an 802-subject placebo-controlled multicenter randomized clinical trial. We target a population of infants for whom the clinical decision to use or not use EA is most challenging in 2018. We will enroll clinically stable ELBW infants with gestational age ?28 weeks, excluding those at high risk for EOS (e.g. suspected exposure to intraamniotic infection) and those at low risk for EOS (e.g. delivery via uncomplicated caesarean section for maternal indications). The aims of the study are (1) To test the hypothesis that the composite incidence of LOS, NEC, or death in infants that receive EA is significantly different than the incidence among infants that receive placebo, (2) To test the hypothesis that fecal samples in the first month of life from infants receiving EA will contain lower diversity and higher abundance of pathogens than fecal samples from infants receiving placebo, and (3) To compare somatic growth (weekly weight and length z-scores) in infants receiving EA and infants receiving placebo (exploratory aim). The results from this study may validate current clinical practice patterns regarding antibiotic administration, or they may provide a critical rationale for further reducing antibiotic usage in the NICU. In addition to improving outcomes for individual babies by reducing antibiotic exposure, such a change could mitigate the growing worldwide burden of antibiotic resistance and nosocomial infections.